Tag Archives: CMS

Today’s announcement at CERN, that the CMS and ATLAS experiments have found a boson consistent with the Standard Model Higgs, is the most exciting find in particle physics since … well, since I can remember. The discovery of charm was before my time, but I was a physics student when news of the W and Z discoveries was made public and I don’t believe those announcements matched today’s press conference for drama and sheer emotion (Peter Higgs had to wipe away a tear).

This is a tremendous day for science. Just think what’s happened here. Over a period of decades, theorists and experimentalists developed a theory of the basic interactions (electromagnetism, weak force, strong force) that govern the behaviour of the fundamental particles (quarks, neutrinos, electron, muon and tau). But in order for the theory to match the observed fact that the fundamental particles have mass, theorists had to add something else into the mix. They used purely mathematical reasoning to deduce something incredible about the Universe: that it’s filled everywhere with a scalar field — the so-called Higgs field. It’s the interaction with this field that gives the fundamental particles mass.

And decades after theorists postulated the existence of this field, CMS and ATLAS have found evidence for the associated boson. They saw hints of the Higgs boson last year. Now it’s definite. It has a mass of around 125 GeV.

This is tremendous news for CMS, ATLAS, CERN and science in general. And it’s the start of a whole new era in physics. Now we know where the Higgs is, the LHC — such a tremendous machine — will be able to investigate its properties in detail. And perhaps for the first time we’ll get a glimpse beyond the Standard Model.

In December 2012 the ATLAS and CMS teams at the Large Hadron Collider announced that they had seen signals that were consistent with there being a Higgs boson with a mass somwhere in the region of about 124-126 GeV. Statistically, though, they were unable to claim a discovery.

Before Fermilab’s Tevatron collider ceased operations in September 2011 its two experiments – CDF and DZero – generated vast amounts of data that have only now been analysed. On 7 March 2012, scientists announced the results of that analysis at the Rencontres de Moriond conference. The data hint at a Higgs boson with a mass somewhere in the range 115-135 GeV. Again, the statistics fall far short of that required to claim a discovery.

The Tevatron collider at Fermilab, as seen from the air. The main ring and main injector are clearly visible. The ponds are there to dissipate waste heat from the machine.Credit: Fermilab, Reider Hahn

This is tantalising! The ATLAS and CMS teams both make use of high-energy proton-proton collisions produced by the LHC, but they are quite different experiments focusing different things. The CDF and DZero experiments are different again: the Tevatron produced proton-antiproton collisions. So a variety of signals are pointing to a Higgs with a mass somewhere around 125 GeV. But there’s no certainty that it’s there: further data might cause the signal to vanish like the Cheshire Cat.

One thing is certain: by the end of 2012 we will know whether the Higgs exists and, if it does, what its mass is. The LHC is operating so well that there’s now nowhere left for Higgs to hide.

I’ve just spent the afternoon watching CERN’s live webcast of the latest CMS and ATLAS data (thank you, CERN, for inventing the Web!). After today there’s very little room for the Higgs still to hide.

ATLAS essentially rules out the existence of a Higgs boson, unless the Higgs mass is in the region 115 to 131 GeV. (I guess I should say that, for comparison, the proton mass is about 1 GeV (actually 0.938 GeV)). CMS seems to rule out a Higgs that is more massive than 127 GeV.

What is tantalising is that both experiments saw hints of a Higgs at around about 125 GeV. Unfortunately, the signal was not strong enough to claim a discovery: what they saw might have been a statistical fluke.

The only way to decide the matter is to take more data which is, of course, what the two experiments will do. In a few months time we will know one way or the other. Either the bumps that ATLAS and CMS saw will go away, and we can say that the Higgs doesn’t exist. Or the bumps will get larger and clearer, and we can say that the Higgs exists with a mass of around 125 GeV.

Either way, new physics will be required. Either way, it will be the discovery of the century.